Development of the auditory receptors of the rat: a SEM study

Fetal and postnatal ontogenesis of the rat cochlea, from the 16th gestational day (16DG) until 3 months post partum, were studied using scanning electron microscopy with emphasis on the stereocilia during the earliest stages of development. The epithelium of the cochlear duct in 16DG rat consisted of plygonal cells topped with numerous microvilli and one central kinocilium, which form the so-called Kölliker's organ. Inner hair cells (IHCs) appeared at 18DG in the basal cochlea. They were characterized by tufts of cilia of the same height and with a kinocilium. The first outer hair cells (OHCs) can be seen at 20DG. The earliest stages of ciliary differentiation, at 18DG for IHCs and 20DG for OHCs, were similar on both types of cells and were characterized by the presence of round bundles of cilia arising from the surrounding microvilli. A three-dimensional V-shaped organization for OHCs and the linear arrangement for IHCs appeared by the end of the first postnatal week, accompanied by the disappearance of transient cilia on the modiolar side of the hair cell and the kinocilium on the external side. The apical pole of OHCs reached adult-like morphology before that of IHCs. Various links between stereocilia were detected already at birth. Morphometric analysis showed that auditory cells from the base of the cochlea reached adult size by the end of the first postnatal week while those from the apex increased their size later. A review of the literature including comparative observations across species on the ontogenesis of the stereocilia shows that hair cells of the stato-acoustic system may present the same early ontogenesis.